1. Field of the Invention
The present invention relates to a directional microphone arrangement having a plurality of microphones, the outputs of which are phase shifted and additively or substractively combined.
2. Description of the Prior Art
Microphone arrangements are known in the art which include a plurality of discrete directional microphones, the outputs of these microphones being phase shifted relative to each other and combined by adding or subtracting. Examples of such known arrangements are described in combination with a zoom microphone in U.S. Pat. Nos. 4,399,327, and 4,412,097, and in the article "Zoom-Mikrofon," Funkschau, 1980, No. 19, pages 79-80. These known directional microphone arrangements have at least three discrete microphones which are individually directional (cardioid pattern) in design. Such microphones are relatively bulky, complex and expensive. The directional discrete microphones are disposed with respect to each other so that two microphones are always directed in the direction toward the sound source, whereas a third microphone is directed away from the sound source. The outputs of the three microphones are additively and/or subtractively combined by potentiometers. By means of the potentiometers, the signal voltage of the discrete microphone directed away from the sound source can be driven to zero. A 180.degree. phase shifter is also used for offsetting the sound signals incident from the lateral directions by 180.degree. relative to each other, so that these signals are cancelled in the adder stage. This results in a directional pattern only toward the front of the arrangement, i.e., in the direction toward the sound source.
A directional microphone arrangement is also disclosed in German OS No. 22 36 968 for a hearing aid which includes at least two sets of microphones, each set including at least two microphones. One set of microphones is disposed at the distal end of one bow of a pair of eyeglasses, and the second set is arranged at the distal end of the second bow of the pair of eyeglasses to be worn by a hearing-impaired person. The microphones within a set are disposed such that their respective directions of maximum signal reception are perpendicular to each other. In this known system, as in the systems described above, microphones which are individually designed as directional microphones are used. The outputs of the discrete microphones are additively and/or subtractively combined by weighting elements. The weighting elements differently weight the sound signals incident at the front and at the sides of the arrangement so that the influence of the sound signals incident at the sides of the arrangement is reduced to a minimum in comparison to the signals incident at the front of the arrangement A front directional effect is thus achieved.
Another directional microphone arrangement is disclosed in U.S. Pat. No. 4,334,740 (corresponding to French Published Application No. 24 36 416 and German OS No. 29 36 082). This arrangement is suitable for cameras and employs integrators having a transfer constant. The transfer constant, however, decays toward high frequencies, so that additional low pass filters and high pass filters are required for reconstructing the higher frequencies.